In general, the use of implantable medical sensors in a patient is known. One example for an implantable sensor is disclosed in U.S. Pat. No. 4,815,469 (Cohen et al.) incorporated herein by reference. The disclosure is directed to an implantable medical sensor which determines the oxygen content of blood. The sensor includes a miniaturized hybrid circuit that includes light-emitting diode means, phototransistor means, and a substrate to which the light-emitting diode means and phototransistor means are bonded in a desired circuit configuration. The hybrid circuit is hermetically sealed within a cylindrical body made from a material that is substantially transparent to light, such as glass. Feedthrough terminals provide means for making an electrical connection with the hybrid circuit. The light-emitting diode means is driven with a stair-stepped current pulse. The purpose of the sensor is to sense the reflective properties of body fluid, such as blood, for spectrophotometric analysis. In one embodiment, the sensor is embedded within a bilumen pacemaker lead and positioned near the distal electrode of the lead so that the sensor resides within the heart when the lead is implanted within a patient, thereby allowing the sensed oxygen content of the blood within the heart to be a physiological parameter that can be used to control the pacing interval of a rate-responsive pacemaker.
U.S. Pat. No. 5,353,800 (Pahndorf et al.) discloses an implantable pressure sensor lead having a hollow needle adapted to be screwed into a patient's heart. The pressure sensor is supplied electrical power through conductors in the sensor.
There are cases where permanent positioning of the sensor is needed. One such case, for example, is disclosed in U.S. Pat. No. 5,404,877 (Nolan et al.), which is incorporated herein by reference. A leadless implantable cardiac arrhythmia alarm is disclosed which continuously assesses a patient's heart function to discriminate between normal and abnormal heart functioning and, upon detecting an abnormal condition, generates a patient-warning signal. The alarm is capable of sensing impedance measurements of heart, respiratory and patient motion and, from these measurements, generating an alarm signal when the measurements indicate the occurrence of a cardiac arrhythmia. It is important to note that the sensor uses an antenna system having a coil inductor for generating an electromagnetic field into tissue for detecting changes in impedance which relate to a physiological phenomena. For example, the size of the inductor is preselected in order to match the dimensions of the organ or structure to be measured.
There are also several known implantable devices that employ telemetry for transmitting or receiving data from an external device. One such device is, for example, the system disclosed in U.S. Pat. No. 6,021,352 (Christopherson et al.). The device utilizes a pressure sensor as a transducer for sensing respiratory effort of the patient. Respiratory waveform information is received by an implantable pulse generator (IPG)/simulator from a transducer and inspiration synchronous simulation is provided by the IPG.
One other telemetric implantable device is disclosed in U.S. Pat. No. 5,999,857 (Weijand et al.). This reference discloses a telemetry system for use with implantable devices such as cardiac pacemakers and the like, for two-way telemetry between the implanted device and an external programmer. The system employs oscillators with encoding circuits for synchronous transmission of data symbols in which the symbols form the telemetry carrier. The system provides circuits for higher density data encoding of sinusoidal symbols, including combinations of BPSK, FSK, and ASK encoding. Embodiments of transmitters for both the implanted device and the external programmer, as well as modulator and demodulator circuits, are also disclosed. It is important to note that the implant device has its own power supply in the form of a battery for powering all of the circuitry and components of the implanted device.
It is also important to note, that to date, there has not been any telemetric medical system that is both a highly efficient system due to its components and their ease of use while providing extremely accurate information regarding a measured parameter in a patient's body.